Regelation Infiltration, Basal Slip and Bed Deformation Neal R
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Geological and Atmospheric Sciences Publications Geological and Atmospheric Sciences 7-2007 Soft-bed experiments beneath Engabreen, Norway: regelation infiltration, basal slip and bed deformation Neal R. Iverson Iowa State University, [email protected] Thomas S. Hooyer University of Wisconsin-Madison Urs H. Fischer Antarctic Climate and Ecosystems Cooperative Research Centre Denis Cohen Iowa State University, [email protected] Peter L. Moore IFoowlalo Swta tthie Usn iaverndsit ay,dd pmooritione@ial wasorktate.se duat: http://lib.dr.iastate.edu/ge_at_pubs Part of the Geomorphology Commons, Glaciology Commons, and the Sedimentology See next page for additional authors Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ ge_at_pubs/144. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Geological and Atmospheric Sciences at Iowa State University Digital Repository. It has been accepted for inclusion in Geological and Atmospheric Sciences Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Authors Neal R. Iverson, Thomas S. Hooyer, Urs H. Fischer, Denis Cohen, Peter L. Moore, M. Jackson, G. Lappegard, and J. Kohler This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ge_at_pubs/144 Journal of Glaciology, Vol. 53, No. 182, 2007 323 Soft-bed experiments beneath Engabreen, Norway: regelation infiltration, basal slip and bed deformation N.R. IVERSON,1 T.S. HOOYER,2 U.H. FISCHER,3 D. COHEN,1 P.L. MOORE,1 M. JACKSON,4 G. LAPPEGARD,5 J. KOHLER6 1Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, 50011, USA E-mail: [email protected] 2Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin, 53705, USA 3Antarctic Climate and Ecosystems CRC and Australian Antarctic Division, Box 252-80, Hobart, Tasmania 7001, Australia 4Norwegian Water Resources and Energy Directorate (NVE), PO Box 5091, NO-0301 Oslo, Norway 5Department of Geography, University of Oslo, PO Box 1042, Blindern, NO-0316 Oslo, Norway 6Norwegian Polar Institute, Polar Environmental Center, NO-9005 Tromsø, Norway ABSTRACT. To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m  1.6 m  0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7–12 days, the glacier regelated downward into the prism to depths of 50– 80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high pore- water pressure (70–100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity. 1. INTRODUCTION Our goal was to avoid these difficulties by building and studying a 1.3 m3 prism of simulated till beneath Enga- The subglacial setting in which temperate ice rests on a soft breen, Norway, (Iverson and others, 2003) where tunnels bed is most important for modulating fast glacier flow and through the subglacial rock provide access to the bed. Our mobilizing basal sediment (Clarke, 2005). Interactions results provide a field demonstration of regelation infiltra- among water, ice and sediment in this environment can tion of ice into a till bed (e.g. Iverson and Semmens, 1995; result in abrupt temporal and spatial variations in glacier Clarke, 2005; Christoffersen and others, 2006), help velocity. On large scales these variations may be mani- illustrate the relationship between basal water pressure fested as surging (e.g. Clarke and others, 1984), spatio- and the coupling between ice and till (e.g. Fischer and temporal switching of ice-stream flow (e.g. Kamb, 2001) Clarke, 2001) and provide new insights into controls on the and lurching behavior of parts of ice sheets that has been vertical distribution of bed deformation (e.g. Boulton and measured directly (Bindschadler and others, 2003) and others, 2001). Associated analysis indicates that regelation inferred from seismicity (Ekstro¨m and others, 2003, 2006). infiltration occurred at rates in agreement with theory, and Also, very high rates of basal sediment transport have been a new model of ploughing clarifies how changes in the postulated for glaciers that are soft-bedded and warm- thickness of a water layer at the bed surface control the based (Hooke and Elverhøi, 1996; Dowdeswell and basal flow mechanism. Siegert, 1999). Moreover, under these conditions some of the most poorly understood of glacial landforms, such as drumlins (e.g. Boulton, 1987) and megascale lineations 2. BACKGROUND (e.g. Ottesen and Dowdeswell, 2006), are thought to have developed. 2.1. Regelation infiltration The most detailed observations of soft-bed processes In a process that Clarke (2005) calls regelation infiltration, beneath glaciers have been made with instruments temperate ice may move downward into the pores of inserted through boreholes drilled to the bed, from either subglacial sediment by melting and refreezing. Evidence for ice tunnels (e.g. Boulton and Hindmarsh, 1987) or glacier this process comes from laboratory experiments (Iverson, surfaces (e.g. Blake and others, 1992; Kamb, 2001). These 1993; Iverson and Semmens, 1995). Its rate is controlled by observations, although illuminating, have been limited by the bed-normal pressure gradient across the sediment layer the restricted range of instruments that can be inserted that has been entrained, which depends on the effective through boreholes. Moreover, interpretations are compli- pressure at the bed and the thickness of the layer. cated by ambiguities regarding instrument position, bed Although the isotopic composition (Iverson and Souchez, geometry and bed disturbance by hot-water drilling. 1996) and structure (Truffer and others, 1999; Christoffersen 324 Iverson and others: Soft-bed experiments beneath Engabreen velocity to seemingly insignificant changes in stress (Bindschadler and others, 2003) indicates that even a small decrease in flow resistance caused by slip over a soft bed could affect flow dynamics over wide areas. The question of where most basal motion occurs and how it depends on water pressure still lacks a definitive answer, in part due to ambiguity in interpreting existing borehole data. For example, Tulaczyk and others (2000a) argued that up- glacier rotation of tiltmeters in basal till reflected expansion of till as pore-water pressure rose, rather than elastic relaxation in shear and associated slip of ice across the bed surface. Also, the processes operating at the bed surface are unclear. Movement there is assumed to occur by a combination of ice sliding past bed clasts that protrude upward into the glacier and ploughing of such clasts across the bed surface (Brown and others, 1987; Alley, 1989; Iverson, 1999). These models, however, have considered basal ice to be clean. If ice generally intrudes into sediment Fig. 1. Site of the till prism, access points to the bed of Engabreen pores, so ice-infiltrated sediment slides over the bed surface, and the bed morphology. The ice tunnel melted to the site of the till the physical relevance of these models is dubious. prism was started from the tunnel access to the bed. The vertical shaft was the site of the sliding velocity measurement. Contour 2.3. Profile of bed deformation interval 1 m. Some data indicate that when the bed shears, rates of shear strain generally increase upward toward the bed surface (Boulton and Hindmarsh, 1987; Boulton and Dobbie, 1998; and others, 2006) of some basal ice facies are consistent Boulton and others, 2001), so the velocity profile is with sediment entrainment by regelation infiltration, it may concave-down. Early discussion attributed this strain pattern be prevented or inhibited beneath glaciers. In fine-grained to an ad hoc rheological rule for till in which the rate of sediments interfacial-curvature and pre-melting effects shear deformation was assumed to be directly proportional (O’Neill and Miller, 1985; Rempel and others, 2004) can to the shear stress and inversely proportional to the effective hold water sufficiently tightly in the pore space to inhibit pressure (Boulton and Hindmarsh, 1987). Since effective refreezing there. Alley and others (1997) estimated that pressure, and hence shearing resistance, increases more regelation should be impeded for sediment grains smaller rapidly with depth than shear stress – at least in the absence than fine to medium silt. In addition, regelation infiltration of upwardly flowing pore water – shear-strain rates, may be inhibited by sliding of ice across the bed, with according to this rule, should decrease downward (Alley, frictional heat impeding refreezing (Alley and others, 1997). 1989). However, diverse laboratory studies do not support Field measurements have not yet demonstrated that this rheological rule (Kamb, 1991; Iverson and others, regelation infiltration occurs. Studying this process subgla- 1998; Tulaczyk and others, 2000a). These studies, in cially is important because it may provide a general agreement with most geotechnical literature, indicate that mechanism of sediment entrainment for wet-based glaciers the most appropriate first-order model for till is that of a (e.g.